1. Field of the Invention
The present invention relates to a limit switch which is normally maintained in a closed position so that an electrically rotatable device, such as a valve, is moved in response to an electrical control signal. The limit switch is tripped at a predetermined position of the valve to open the circuit, and to thereby interrupt a supply of power to the movable device.
2. Discussion of the Background
As described in U.S. Pat. No. 4,939,320 to Graulty, it is a conventional practice to use micro or limit switches to control the positioning of a rotatable valve at a selected one of its opened and closed conditions. Typically, two angularly displaced cams are integrally formed or rigidly attached on a shaft which is coupled for rotation with a rotatable stem of a valve. As the shaft rotates in either a clockwise or counterclockwise direction with the valve stem, the actuating cams will move either clockwise or counterclockwise along a path which brings one or the other of the cams into contact with one or the other of the limit switches, thereby interrupting a supply of electrical current to a motor driven valve, and stopping movement of the valve at either its open or closed position. The same cams and limit switches can be used to turn on or turn off status signals which represent the open or closed status of the motor driven valve.
However, as also discussed in the patent to Graulty, most of the prior art arrangements require complicated procedures to set up the angular orientation of the cams relative to their mounting shafts to ensure actuation of the limit switches at the desired position of the valve being controlled. Also, the setup must periodically be examined, and readjusted if necessary.
The patent to Graulty discloses an invention including cam members which are frictionally mounted on a rotating shaft. In a first embodiment of the Graulty invention, the cam members are preferably composed of a plastic material while the shaft is made of a metal material. However, in high temperature or low temperature environments, the friction fit of the parts is altered due to their different thermal expansion coefficients. Thus, the Graulty invention is only suited to a narrow range of temperatures. Although Graulty discloses a second embodiment of his invention for use in high temperature environments, a complex arrangement of springs, washers, annular shoulders, and nuts is necessary to supply a frictional force sufficient to cause the cam members to rotate along with the shaft.
Consequently, a need exists for a simplified cam/shaft interface which supplies an appropriate frictional force between the shaft and the cam at a wide range of temperatures.
Additionally, a need exists for an apparatus which can easily be assembled and disassembled without a need for special tools, and which can be calibrated by manual adjustment after complete assembly.